1. Field of the Invention
This invention relates to the production of fluoride-free phosphoric acid, and optionally substantially fluoride-free potassium dihydrogen phosphate, by the acidulation of phosphate rock with sulfuric acid, and more particularly to a process comprising the acidulation of phosphate rock with sulfuric acid wherein the reaction is conducted in the presence of a controlled amount of potassium ion as K.sub.2 O and a controlled amount of silica as SiO.sub.2.
2. Description of the Prior Art
Phosphoric acid plants are currently operated utilizing a basic and well known process for the acidulation of phosphate rock which comprises reaction of the rock with sulfuric acid to form phosphoric acid with subsequent reaction of the phosphoric acid with ammonia to produce ammonium phosphates. The phosphoric acid formed in this process is called wet process phosphoric acid. In this reaction, a by-product is gypsum having the chemical formula CaSO.sub.4.2H.sub.2 O. Essentially, all phosphate rock contains some fluorine normally in the 3.0-4.0 percent range and the acidulation reaction usually generates gaseous fluorides which in prior years was usually evolved into the atmosphere or trapped with water scrubbing apparatus.
In recent years, air and water pollution law regulations have become more stringent and are now being enforced vigorously. Therefore, operators of phosphoric acid plants have had many pollution problems with fluorine emission in the atmosphere and with the by-product gypsum from these phosphoric acid plants. An important problem in the operation of these wet process phosphoric acid plants has been in the expensive methods necessary for handling the large amounts of fluorine compounds liberated in the gaseous and aqueous effluents from such plants. In some phosphate complexes from 10,000 to 30,000 tons per year of fluorine compounds may be liberated by various methods and it is estimated that in a typical wet process phosphoric acid plant, a portion of the fluorides are evolved in the atmosphere in gaseous form such as hydrogen fluoride and silicon tetrafluoride which can destroy vegetation and affect other facilities in close proximity to the plant if they are not scrubbed out, and such scrubbing systems are not always effective. A second portion of the fluorine is found in gypsum ponds and is subject to reaching into groundwater and streams. Still another portion of the fluorine remains with the final products and when such final products are used as fertilizers they may introduce fluorine into the soil. Only in recent years have studies been made on the effects of fluorides contained in final products and indications seem clear that they may have a deleterious effect on the long range producing ability of the soil, see for Example Kudzin et al., Chemical Abstracts, 73, 870534 (1970).
There is a great deal of literature and patent art related to attempts to remove the fluoride values from fluorine-containing phosphate rock in the operation of a phosphoric acid plant including methods for suppressing the evolution of fluoride values in the operation of a process and/or attempting to scrub the fluorine from effluent gases and waste water. Two such methods are described in U.S. Pat. Nos. 2,954,275 and 2,976,141 to Carothers et al. which use sodium or potassium compounds to suppress the fluorides so that they are concentrated in the gypsum cake. These patents indicate that this is achieved by adding a suppressing amount of an alkali metal salt to the acidulation reaction. However, these processes were conducted in the presence of sulfuric acid in the acidulation reactor and the process had incomplete control on fluoride decomposition and evolution during acidulation.
Other prior art has been noted which attempts to overcome the problem of fluorine evolution and a reduction in the amount of fluorine contained in final products. A reference of this type is British Pat. No. 735,086 (1955), which discloses the acidulation of phosphate rock by a two step procedure using a strong mineral acid such as nitric acid or hydrochloric acid. According to this patent, an initial low temperature acidulation at 20.degree.-50.degree. C. is carried out with the addition of an alkali, for example, ammonia or lime, as a precipitating agent in a quantity sufficient to precipitate substantially the whole of the fluorine and other impurities but insufficient to precipitate a substantial amount of calcium phosphates.
In a similar process in U.S. Pat. No. 3,431,096 to Hill et al., a process is disclosed for reducing evolution of fluorine values in formation of triple superphosphate fertilizer by reaction of phosphate rock and phosphoric acid wherein ammonia or urea is added to suppress the fluorine evolution. However, in this patent, there is no provision for removal of the fluorine values from the product and therefore even if the fluorine evolution is prevented, the fluorine values will be retained in the resulting product and therefore distributed to the soil when it is used as a fertilizer.
In a series of patents issuing from the mid 1940's to early 1960's there are disclosed processes for the defluorination of phosphate rock and the production of defluorinated calcium phosphates. In these patents, U.S. Pat. Nos. 2,337,498; 2,442,969; 2,893,834; and 2,997,367, the defluorination reaction is carried out by subjecting a mixture of phosphate rock, phosphoric acid and an alkali metal material to calcination, that is by reaction at temperatures as high as 1000.degree. C. to 2200.degree. C. Obviously, under these conditions the fluorine is going to be rapidly evolved or if not evolved, certainly will remain in the final product, said to be an animal feed.
Two additional patents of pertinence to processes of this type are U.S. Pat. Nos. 2,567,227 and 2,728,635 to Miller which disclose the acidulation of phosphate rock with phosphoric acid to form monocalcium phosphate, cooling to solidify the monocalcium phosphate and then converting it to dicalcium phosphate by hydrolysis. In the earlier patent, it is indicated that the fluorine in the rock is vaporized in the system, circulates throughout the system and/or leaves the system with the calcium phosphate. The later patent indicates that the process of U.S. Pat. No. 2,567,227 provided a final calcium phosphate product having a fluorine content too high to be of animal feed grade. The solution to this problem in the later patent was the addition of some dilute sulfuric acid in the acidulation step which would of course lead to additional fluorine evolution during the first step.
There are also patents known in the art which indicate that it is known to acidulate phosphate rock with phosphoric acid and to then recover solid monocalcium phosphate by cooling of the resulting solution and recovering the monocalcium phosphate. Processes of this type are disclosed for example in U.S. Pat. Nos. 3,494,735 and 3,645,676. In addition, U.S. Pat. Nos. 3,619,136 and 3,792,151 to Case disclose the reaction of phosphate rock with recycle phosphoric acid at temperatures of about 125.degree.-180.degree. F. (52.degree. C. to 83.degree. C.) to form a solution of monocalcium phosphate, reacting the latter solution with sulfuric acid to produce phosphoric acid and calcium sulfate, precipitating the calcium sulfate, and recycling a portion of the phosphoric acid to the phosphate rock reaction. These patents point out that under the conditions recited, fluorides are not evolved but remain primarily unreacted and may be found with insoluble materials although a portion remains in the phosphoric acid solution product. It is also known to react phosphate rock or a solubilized form with sulfuric acid and KHSO.sub.4 in combination with other steps and this reaction is described in U.S. Pat. Nos. 3,697,246 and 3,718,253.
In copending application Ser. No 608,973, filed Aug. 29, 1975, of one of us, there is disclosed a process for the acidulation of phosphate rock and production of substantial pure alkali metal phosphates, calcium phosphates and phosphoric acid which comprises primarily the steps of acidulating phosphate rock with a phosphoric acid solution containing sufficient alkali metal values to provide potassium ions in the system and thus form an insoluble precipitate comprising a mixture of impurities, silicas and fluorides from which the fluorides can be recovered in usable form. It is a feature of this disclosure that the fluorides are not evolved into the atmosphere but are primarily recovered in the insoluble precipitate removed prior to gypsum precipitation. In addition, in prior U.S. Pat. No. 3,840,639 there is disclosed a process for the acidulation of phosphate rock by reaction of the rock with phosphoric acid in the presence of potassium ion.
Of other pertinent art in this area, U.S. Pat. No. 2,114,600 to Larson discloses the reaction of phosphate rock with nitric acid in order to form dicalcium phosphate and calcium nitrate. The patentee points out that in this system difficulties are experienced because the fluorine present in phosphate rock is precipitated as a very slimy calcium fluoride extremely difficult to separate from the nitrate solution and to overcome this problem, the patentee suggests that the phosphate rock be dissolved in the nitric acid in the presence of a fairly large amount of silica while at the same time adding an alkali salt such as alkali nitrate or chloride so that the fluorine reacts with the silica and alkali salt to provide an insoluble well crystalized alkali silicofluoride which can easily be separated from the solution nitrates and phosphoric acid. The patent, however, is limited to the addition of large excesses over the theoretical amount of alkali salt.
A second patent in this area is U.S. Pat. No. 2,865,709 to Horn et al which relates to the production of insoluble silicofluorides wherein phosphate rock is mixed with sulfuric acid and the gas evolved from the system containing silicon tetrafluoride is absorbed in water to form a waste liquor solution containing fluosilicic acid. When this solution is mixed with a chloride such as potassium chloride, insoluble fluosilicates are formed but the silica fluoride develops into a highly dispersed state as an unfilterable gel. This patentee proposes to overcome this problem by adjusting the fluorine to silica mol ratio in the waste liquor so as to have a mol ratio of fluorine to silica of about 4.4:1 before reaction with potassium chloride.
The present invention provides an acidulation system which substantially eliminates the problem of fluorine evolution in the acidulation of fluorine-containing phosphate rock with phosphoric acid and also provides systems wherein substantially pure phosphoric acid is recovered, and the fluoride contained in phosphate rock and phosphoric acid is precipitated as potassium silicofluoride. Therefore, the present invention provides a unique combination of steps and advantages not appreciated heretofore in the prior art as none of these prior references disclose the unique combination of steps and results of this invention.